Clathrates are solid cage-like structures in which hydrogen bonding between molecules forms a lattice of host molecules that trap and contain other molecules termed guest molecules. For example, a gas clathrate hydrate is composed primarily of water molecules that form a lattice enclosing and trapping a gas.
Clathrates typically form at low temperature and high pressure.1 Guest molecules (in addition to a gas) can stabilize the clathrate to higher temperatures and pressures.2 1 Raman Spectroscopic Investigation of CO2 Clathrate Formation, a Thesis, Patric Benziher, Ohio State University 2004.2 “Hydrogen Clusters in Clathrate Hydrates”, Mao, W. L. et al, Science, Vol. 297, pages 2247-2249 (2002).
Different structural arrangements of clathrates are distinguished by their unit cells—i.e., repeating structures of their respective lattice networks.
One clathrate structure, termed “sII”, is well studied. See, for example, “Inorganic Clathrates for Hydrogen Storage”, Struzhkin et al, Hydrogen Program Annual Review, DOE 2005 Carnegie Institute Washington. The sII unit cell is made up of 16 pentagonal dodecahedrons and 8 pentakaidecahedrons, respectively designated the small cage and large cage. Typically, large guest molecules occupy the large cage, leaving the small cage for gas storage. Struzhkin et al. cited above. The large proportion of 512 cavities in sII clathrates is thought responsible for the similarities in the Raman spectra to gas saturated water: the sII clathrate has a peak in Raman analysis at the 3100 cm−1 region. See, Mao et al., cited above.
U.S. Pat. No. 6,735,960 (hereby incorporated by reference in total) includes figures (FIGS. 5A, 5B, and 5C) depicting an sII crystal clathrate structure. The structure is characterized by cubic crystals containing sixteen 512 cavities, eight larger 51264 cavities. The tetrahedral 51264 cavities form an open tetrahedral network, with their centers arranged in a structure analogous to that of cubic ice, separated by groups of three 512 cavities. FIG. 5B of '960 shows four hydrogen molecules in one of the larger 51264 cavities and FIG. 5C of '960 shows two hydrogen molecules in one of the smaller 512 cavities. The unit cell contains 136 H2O molecules and 64 H2 molecules for a ratio of hydrogen/water=0.47. The '960 patent reports that the sII clathrate structure vanished above 115° K (at 10 kPa), but, at higher pressure (200 Mpa), the structure is stable to 280° K.
A second clathrate structure, termed “sI”, exhibits a long broad peak above 3000 cm−1 in Raman spectrometry for the O—H stretching mode.3 In the sI structure, linear tetrakaidecahedral (51262) cavities form three orthogonal axes holding a dodecahedral cavity wherever they cross (ratio 6:2 respectively per unit cell); each dodecahedral cavity sitting (in a body-centered cubic arrangement) within a cube formed by six tetrakaidecahedral (51262) cavities. These (51262) cavities join at their hexagonal faces to form columns. 3 “In Situ Observations of Methane Hydrate Formation Mechanisms in Raman Spectroscopy”, Uchida et al, Annals New York Academy of Sciences, p 593-601 (DATE?).
A third clathrate structure is termed “sH”.
The following tables present some clathrate properties.4 4 http://www.lsbu.ac.uk/water/clathrat2.html.
Characteristic properties of the clathratesSpaceTypeLatticegroupUnit cellUnit cell formula5Clathrate ICubicPm3na = 1.20 nm(S)2•(L)6•46H2OClathrate IIFace-Fd3ma = 1.73 nm(S)16(L+)8•136H2OcenteredcubicClathrate HHexagonalP6/mmma = 1.23 nm(S)5(L++)•34H2Oc = 1.02 nm5Not all cavities would normally be filled; S = small guest; L = large guest; L+ = larger guest; L++ = largest guest
Clathrate Unit CellsCavity512512625126451268435663H2O2024283620Mean cavity3.954.334.735.714.06radius, Åfree volume,517712021344Å3Clathrate26———I,/unit cellClathrate16—8——II,/unit cellClathrate3——12H,/unit cellGuestAr, O2,CO2,C3H8,(CH3)3CC2H5CH4molecules,N2, CH4C2H6(CH3)3CHe.g.1.8-2.21.8-2.72.8-3.13.5-4.31.8approximateradius, Å
Feil et al, The Polyhedral Clathrate Hydrates. Part 2. Structure of the Hydrate Tetra Iso-amyl Ammonium Fluoride, Journal of Chemical Physics, Vol 35 No 5 November 1961 review various gas/clathrate structures including a hydrate of tetra iso-amyl ammonium fluoride. The authors postulate that anions and cations substitute for the oxygen in the clathrate structure.
In addition to the '960 patent discussed above concerning storage of H2 in an sII clathrate, there are other reports of clathrate gas storage. Storage of H2 in an SII clathrate is disclosed in “Inorganic Clathrates for Hydrogen Storage”, Struzhkin et al, Hydrogen Program Annual Review, DOE 2005 Carnegie Institute Washington. Florusse et al. Science Vol. 306, pp. 469-471 (2004) discloses hydrogen storage in a binary SII clathrate hydrate with tetrahydrofuran (THF).
Uchida et al, report6 that methane forms an sI clathrate, although methane hydrates are generally thought to decompose at standard conditions making recovery difficult. The authors conclude, 6 “In Situ Observations of Methane Hydrate Formation Mechanisms in Raman Spectroscopy”, Uchida et al. Annals New York Academy of Sciences, p 593-601 (2004)                Methane hydrate (CH4.nH2O) is a crystalline molecular complex that includes a large quantity of methane molecules and is stable at high pressure and low temperatures. Its unit cell consists of 46 water molecules that construct two small cages (pentagonal dodecahedron, 512) and six large cages (tetrakaidecahedron, 51262) [Citing Sloan, Ed., Jr., 1998. Clathrate Hydrate of Natural Gases, 2nd edit. Marcel Dekker Inc., New York]. Since at most one CH4 molecule can occupy each cage, eight or fewer CH4 molecules are expected per unit cell. If the hydrate is fully occupied by CH4 molecules, the number of moles of water reacted with one mole of CH4, n, is 5.75.They observed Raman spectra due to hydrate formation at 264.2° K (−8.95° C.) and 6.3 MPa. (62.18 standard atmospheres)        
In a May 16-19 2006 Program Review, Rovetto et al. of the Colorado School of Mines Center for Hydrate Research report storage of H2 in sII clathrates stabilized by tetrahydrofuran, cyclohexanone, tetra-n-butylammonium bromide. They also reference a methane p-toluene sulfonic acid structure